1. Field of the Invention
The present invention relates to an image forming apparatus including a drum unit and a developing unit for transcribing an image onto a record sheet, and particularly to a link mechanism a linking a lower frame and an upper frame of the apparatus together for making exchange of the drum unit and/or the developing unit easy.
2. Description of the Related Art
Recently, an image forming apparatus is widely used for transcribing an image onto a recording sheet as seen in a duplicator or a facsimile apparatus and tends to be small in size and light weight.
In such image forming apparatus, a frame is mostly separated into a lower frame and an upper frame for easily exchanging units thereof. However, the units must be exchanged very carefully because the units are delicate and are installed in a small space very close to each other. Furthermore, as the image forming apparatus is used popularly, the units are required to be exchanged by users themselves without asking the help of an expert. Therefore, the image forming apparatus is required to have structure in which the units can be exchanged easily by any person and safely without damaging any unit.
For convenience of exchanging the units, some units are unified in an integrated unit, considering a balance between the lives of the units unified. Particularly, the life of the photoconductive drum, which will be called simply the "drum" hereinafter, is important to maintain a high quality of a transcribed image on the recording sheet. Therefore, the drum is generally unified in the integrated unit called a drum unit, unifying other units associated with the drum.
In the image forming apparatus, an image is transcribed on the recording sheet as follows: an electrical image signal to be transcribed is fed to the image forming apparatus; the drum is rotated and a cylindrical surface of the drum is electrostatically charged by a charging unit; a latent image is formed on the charged cylindrical surface by an optical beam projected from a projecting unit, wherein the optical beam is modulated by the electrical image signal; the latent image is developed by a developing unit, producing a toner image on the cylindrical surface; the toner image on the cylindrical surface is transcribed onto the recording sheet by a image transcription unit; a transcribed toner image on the recording sheet is fixed by a fixing unit; and the toner left on the cylindrical surface is cleaned by a cleaning unit and kept rotating for the next image transcription. The recording sheet is fed to the image transcription unit and sent to the fixing unit by a sheet transferring mechanism. The drum, the charging unit and the cleaning unit are unified to an integrated unit called a drum unit. The drum unit and the developing unit are usually installed in the upper frame, and the image transcription unit and the fixing unit are installed in the lower frame.
In the image forming apparatus operating as mentioned above, a gap between the cylindrical surface and the developing unit and a gap between the cylindrical surface and the image transcription unit are very important for producing a high quality of the recorded image. Therefore, the developing unit and the image transcription unit are placed against the cylindrical surface so as to have a designated constant gap respectively, by inserting rollers belonged to the units respectively, between the cylindrical surface and the units. The rollers will be called gap rollers hereinafter.
As a result, when the developing unit and/or the drum unit is exchanged, the upper frame is opened from the lower frame and then the developing unit and the drum unit are separated from each other so that the gap rollers belonged to the developing unit are parted from the cylindrical surface. In a case of the image transcription unit, the separation process as mentioned above is unnecessary to be performed because the drum unit and the image transcription unit are separated when the upper frame is opened from the lower frame.
In the prior art, when the separation process of the drum unit and the developing unit is performed in the upper frame, the developing unit must be parted from the drum unit by loosening the developing unit manually as disclosed in Japanese laid open Utility Model Application 60-51554, from the upper frame manually. That is, when the drum unit is required to be exchanged for example, the following steps must be carried out in the prior art: firstly opening the upper frame from the lower frame; secondly loosen fastening means such as levers having fastened the developing unit to the upper frame moving the developing unit so that the gap rollers belonging to the developing unit are parted from the cylindrical surface of the drum against a force pushing the developing unit toward the cylindrical surface; and removing the drum unit from the upper frame. These steps have been very inconvenient to carry out for the general user.